1. Field of the Invention
The present invention relates to a clutch assist device which can reduce the force necessary to operate a friction clutch to engage and disengage a transmission of a vehicle.
2. Description of the Related Art
An engine unit of a vehicle, for example a motorcycle, is often equipped with a friction clutch that operates to engage and disengage a transmission. With the clutch engages, the transmission transfers torque from the engine crankshaft to one or more wheels of the motorcycle and, with the clutch disengaged, the transmission is decoupled from the engine so as not transfer a driving torque to the wheel(s). The friction clutch typically has friction plates and clutch plates laid alternately next to one another. A clutch spring presses the friction plates against the clutch plates and biases the friction plates into this state so as to transmit torque.
The clutch has a clutch release mechanism. The clutch release mechanism, which is used to release the pressure against the friction plates caused by the clutch spring, is coupled to a clutch operating lever via a clutch wire. A Bowden-wire cable is often used as the clutch wire and includes an inner wire that slides relative to an outer sheath. The clutch operating lever is usually disposed on the handlebars of the motorcycle. When the rider squeezes the clutch operating lever, the friction plates are separated from the clutch plates, against the urging force of the clutch spring, to disengage the clutch and, hence, the transmission from the engine.
With a friction clutch used in a high-power, high-revolution engine, it is desired to set the biasing load of the clutch spring to a high value to increase the torque capacity of the clutch. However, since the clutch operating lever is operated manually, it often is difficult for at least some riders to operate the clutch operating lever when the biasing load of the clutch spring is increased.
A clutch assist mechanism has been previously proposed to reduce the force necessary to actuate the clutch operating lever. See U.S. Pat. No. 5,495,928. The clutch assist device disclosed in the '928 patent is installed at an intermediate point of a clutch wire that connects a clutch operating lever and a clutch lever of a friction clutch.
The clutch assist mechanism of the '928 patent has an elongated frame secured to the vehicle body. The frame has wire through holes at its longitudinal ends through which the inner wire of the clutch wire extends. The frame has openings on both sides thereof.
A pair of plate springs is attached to the frame. The base ends of the plate springs are secured to the sides of a first end of the frame. The distal ends of the plate springs protrude from the first end of the frame and face the openings of the frame. The clutch assist mechanism also has a pair of linking rods and a clamp bracket. The clamp bracket is secured to the wire extending through the frame and is movable together with the wire. Each linking rod has one end connected to the clamp bracket and the other end connected to the distal end of its corresponding plate spring.
In the operation clutch assist mechanism, when the clutch operating lever is squeezed to disengage the friction clutch, the clamp bracket is pulled toward the first end of the frame, which causes the linking rods to tilt and the plate springs to deform. Then, when the linking rods pass the point where they are perpendicular to the longitudinal direction of the frame, the resultant of the restoring forces of the plate springs acts in the same direction as the direction in which the inner wire is pulled by the clutch operating lever, and the clamp bracket is urged together with the inner wire. As a result, the urging force acts to assist the clutch operating lever to spread the friction plates from the clutch plates against the bias of the clutch spring. Consequently, the manual force required to operate the clutch is reduced.
While the clutch assist mechanism disclosed in the '928 patents reduces the manual force required to operate the clutch operating lever, the mechanism is necessarily long and, thus, requires a frame having a long linear portion. The long length of the clutch assist mechanism is dictated by its structure: the clamp bracket must move in the frame from one end toward the other to generate an assist force. Accordingly, it is difficult to reduce the size of the mechanism, which in turn limits design options for the vehicle (e.g., the motorcycle) on which it is used.
Also, because a long frame must be disposed at an intermediate point of the clutch wire, a relatively long linear space must be additionally formed at an intermediate part of the clutch wire. As a result, another section or sections of the clutch wire must be curved into an arc with a large radius of curvature to transition from a generally vertical orientation to a generally horizontal orientation as the wire is routed from the clutch operating lever (which usually resides on the handlebars) to the clutch (which usually resides near or below the engine and remote to the handlebars).
In general, when the clutch wire is curved sharply, the inner wire rubs against the outer sheath at the curve and a large sliding resistance is applied to the inner wire when the clutch lever is operated. Such friction increases the operating force required to disengage the clutch. Thus, the assisting function of the clutch assist mechanism is reduced or canceled. Consequently, the some riders may find the clutch operating lever hard to actuate.
A need therefore exists for a compact clutch assist device that assists the operation of a clutch lever and that can be easily mounted on a vehicle with improved convenience.